Siemens SIMATIC Operating Instructions Manual page 170

Glossary
Response Time
The response time is the time between detection of an input signal and a change in the
gated output signal.
The actual response time lies somewhere between a minimum and maximum response time.
The maximum response time must always be anticipated when configuring a system.
For fail-safe digital inputs: The response time is the time between a signal change at the
digital input and safe loading of the -> safety message frame on the backplane bus.
For fail-safe digital outputs: The response time is the time between an incoming safety
message frame from the backplane bus and the signal change at the digital output.
Safe State
The basic principle behind the safety concept in F-systems is the existence of a safe state
for all process variables. For the digital F-I/O, for example, the safe state is the value "0".
Safety Class
Safety Integrity Level (SIL) in accordance with IEC 61508 and prEN 50129. The higher the
Safety Integrity Level, the more rigid measures for the prevention of systematic faults and for
the management of systematic faults and hardware failures.
Fail-safe modules can be used in safety mode up to SIL3.
Safety Function
Safety function is a mechanism built into the -> F-CPU and -> F-I/O that allows them to be
used in -> S7 Distributed Safety or S7 F/FH Systems fail-safe systems.
In accordance with IEC 61508: A safety function is implemented by a safety system to
ensure that the system is kept in a safe state or brought into a safe state in the event of a
particular fault.
Safety Message Frame
In safety mode, data are transferred between the -> F-CPU and the -> F-I/O in a safety
message frame.
Safety Mode
Safe mode is the operating mode of the -> F-I/O that allows -> safety-related communication
by means of -> safety message frames.
-> ET 200pro fail-safe modules are designed for operation only in safety mode.
Safety Program
Safety-related user program
170
ET 200pro Distributed I/O System - Fail-Safe Modules
Operating Instructions, 05/2007, A5E00394073-02